Calculate risers, treads, stringer length, and materials for stairs.
Stair Building Reference
Building stairs requires precise calculations for safety and code compliance. Every riser must be the same height and every tread the same depth.
| Component | Standard Size | Code Requirement |
|---|---|---|
| Riser Height | 7-7.5 in | Max 7.75 in |
| Tread Depth | 10-11 in | Min 10 in |
| Stair Width | 36-48 in | Min 36 in |
| Headroom | 80+ in | Min 80 in |
| Handrail Height | 34-38 in | 34-38 in |
Frequently Asked Questions
How do I calculate the number of stairs?
Divide the total rise (floor-to-floor height) by the ideal riser height (7-7.75 inches). For a 9-foot (108-inch) rise: 108 ÷ 7.5 = about 14-15 risers. The number of treads is one less than risers.
What is the standard stair riser height?
Building codes typically require 7-7.75 inch risers. The ideal is 7-7.5 inches for comfort. All risers in a staircase must be within 3/8 inch of each other for safety.
What is the standard stair tread depth?
Minimum tread depth is 10 inches (measured from nosing to nosing). The ideal is 10-11 inches. The ‘rule of 25’ states riser + tread should equal approximately 17-18 inches for comfortable stairs.
How wide should stairs be?
Minimum width for residential stairs is 36 inches (code). 42-48 inches is more comfortable. Main staircases in larger homes are often 48-60 inches. Handrails reduce usable width by about 3 inches per side.
How do I calculate stair stringer length?
Use the Pythagorean theorem: stringer length = √(total rise² + total run²). For 9 ft rise and 12 ft run: √(81 + 144) = √225 = 15 feet. Add 12 inches for cuts and adjustments.
How much does it cost to build stairs?
Basic interior stairs: $1,000-3,000. With hardwood treads: $3,000-8,000. Spiral stairs: $3,000-10,000. Exterior deck stairs: $500-2,000. Custom/curved stairs: $10,000-30,000+.
How many stringers do I need?
Minimum two stringers (one per side). For stairs wider than 30 inches, add a center stringer. For 48-inch wide stairs, use three stringers. Open stairs may need four stringers for extra support.
What lumber is used for stair stringers?
2×12 dimensional lumber is standard for stringers. The board must be wide enough that at least 3.5 inches of wood remains after cutting the notches. For exterior stairs, use pressure-treated lumber.
Do stairs need a landing?
Codes typically require a landing for stairways that rise more than 12 feet vertically. Landings must be at least as deep as the stair width (36 inches minimum). L-shaped and U-shaped stairs use landings to change direction.
What is a winder staircase?
Winder stairs use pie-shaped treads instead of a landing to change direction. They save space but are less safe than landing stairs. Building codes restrict their minimum tread width (6 inches at narrow end).